1. Field of the Invention
This invention relates to apparatus for and methods of analyzing a surface which use a microbeam of particles of electrons, positrons, ions, neutrons, or photons, such as ultraviolet light, vacuum ultraviolet light, X rays, etc., to analyze elements on the surface of a sample in a destructive or nondestructive way.
2. Description of the Related Art
With the recent development of semiconductor fabricating technology, various methods for surface analysis have been noticed as means of evaluating semiconductor materials. In particular, the analyzing technique of examining the state of contamination on the surface of a silicon wafer is indispensable for a semiconductor fabricating process. The most powerful means of evaluation is the method of removing adsorbed atoms and molecules from the surface of a sample, such as the silicon wafer, analyzing the mass of ions produced, and examining the state of contamination of carbon compounds and water molecules of the sample. Specifically, the methods, such as secondary ion mass spectrometry (SIMS), electron stimulated desorption (ESD), and photon stimulated desorption (PSD), are well known as means of spectrometric analysis. For example, when the surface of the sample is irradiated with fast ions, ionized atoms which constitute the surface of the sample are emitted from the surface as secondary ions. The method of analyzing the mass of the secondary ions is the SIMS.
A conceptual view of the SIMS is given in FIG. 1. Among the elements shown in FIG. 1 are: a CRT 100, a scanning power means 101, an optical microscope 102, amplifiers 103, 108, a secondary electron multiplier 104, a screen electrode 105, a sample holder 106, a recorder 107, a secondary ion extracting electrode 109, a collector slit 110, a sector magnetic field 111, a .beta. slit 112, a sector electric field 113, an objective stop 114, a compensating lens 115, and a slit 116. A beam of ions, such as Cs, emitted from an ion gun 1 pass through a condenser lens 2 and an objective lens 3, and is converged into a spot diameter of 1 .mu.m or less on the surface of a sample 4. The ion beam is deflected when traversing a deflection electrode 5 disposed between the condenser lens 2 and the objective lens 3, and thus can be made scan the sample 4 two-dimensionally by controlling the deflection electrode 5. The secondary ions discharged from the sample surface by irradiation with the ion beam pass through a mass spectrometer 6 and are detected at a secondary electron multiplier 7 by a mass spectrometer. If the ion beam is mass-analyzed by means of the two-dimensional scan, a two-dimensional spatial mapping of a specific element will be possible.
The SIMS shown in FIG. 1 is constructed so that a secondary electron and visible light, in addition to the secondary ion, emitted from the sample surface can also be observed, thus providing a multiple analysis. For the spectrometry of the secondary ion, a time-of-flight spectrometry is sometimes used. This method is analogous to an X-ray microanalyzer by using an electron beam as a probe in view of the fact that the microscopic region of the sample surface can be analyzed. However, it is superior to the X-ray microanalyzer because of the facts that the analysis of isotopic elements is possible and that an analysis sensitivity, notably a detection sensitivity to light elements, is higher. A means of scanning and analyzing the sample surface with an electron beam in the arrangement similar to the SIMS as mentioned above is called the ESD. On the other hand, a means of irradiating and analyzing the sample with light of shorter wavelength than ultraviolet light, using a synchrotron radiation (SR) source or a mercury-vapor lamp, is termed the PSD.
Next, reference is made to the mass spectrometry and the time-of-flight spectrometry which are typical secondary ion spectrometries.
For the mass spectrometry, FIG. 2 is a conceptual view showing the mass spectrometer 6. The secondary ions generated from the surface of the sample 4 (refer to FIG. 1) are accelerated by a secondary ion extracting electrode 9 and converged by a compensating lens 10. They travel through an entrance slit 11 and are selected by a coaxial cylindrical sector electrode 12 and a sector magnet 13. Specifically, since the ions passing through the entrance slit 11, incident on the sector electrode 12 have particular velocity, mass, and electric charges, they are deflected so as to follow the orbit of the sector electrode 12 only where a particular voltage is applied to the electrode 12, and can thus pass through the electrode 12. Subsequently, the ions having traversed the sector electrode 12 are selected, through two slits 14 and 15, by the sector magnetic field produced by the sector magnet 13. The sector magnetic field 13 has such behavior that a magnetic force is applied perpendicular to the orbital plane of the secondary ion and the orbit of the secondary ion is changed by the Lorentz force. According to the arrangement depicted in FIG. 2, two slits 15 and 16 are disposed at the entrance and the exit, respectively, of the region in which the sector magnetic field 13 is present. Whereby, when the secondary ions travel through the sector magnetic field 13, an arcuate orbit can be defined. Hence, by choosing a magnetic flux density, the secondary ion having the particular velocity, mass, and electric charge can pass through the slit 16. The ion thus selected is detected by the secondary electron multiplier 7.
For the secondary ion thus available, a description is made of the method of calculating quantitatively the result of the mass analysis. In the sector electrode 12, the intensity of the sector electric field is designated by E and the orbital radius of the sector electrode by R.sub.1, while in the sector magnetic field 13, the magnetic flux density of the sector magnetic field is denoted by B and the orbital radius of the sector magnetic field by R.sub.2. The mass of the incident ion is represented by m, the ionic valence number by n, the electronic charge by e, and the velocity of incidence by u. In this case, the condition of the sector electric field that the secondary ion can follow the arcuate orbit of the radius R.sub.1 is that an electrostatic force balances with a centrifugal force in the sector electric field. This is given by EQU e n E=m u.sup.2 /R.sub.1 ( 1)
Similarly, the condition of the sector magnetic field 13 that the secondary ion can follow the arcuate orbit of the radius R.sub.2 is that the Lorentz force balances with the centrifugal force in the sector magnetic field 13. This is given by EQU e n u B=m u.sup.2 /R.sub.2 ( 2)
Elimination of u from Eqs. (1) and (2) yields EQU e n/m=(R.sub.1 /R.sub.2.sup.2)(E/B.sup.2) (3)
Hence, the mass analysis of an arbitrary ion can be made in accordance with the intensities of the sector electric field and the sector magnetic field.
Next, time-of-flight spectrometry is explained. The time-of-flight spectrometer is simpler in system structure than the mass spectrometer, and its conceptual view is given in FIG. 3. In this figure, a sample 18 is placed in the uniform electric field of an electrode 19 to accelerate the secondary ions generated by the irradiation of a laser beam toward a drawing electrode 20. The ion accelerated to the velocity u by passing through the drawing electrode 20 travels a distance L from the position of the electrode 20 and is then detected by a secondary electron multiplier 21.
Now, reference is made to the method of calculating quantitatively the result of the mass analysis based on the time-of-flight spectrometry. If it is assumed that the secondary ions are accelerated at a voltage V, the relation with the velocity u can be written, by the law of conservation of energy, as EQU e n V=(1/2) m u.sup.2 ( 4)
When the distance between the two electrodes 19 and 20 is taken as k, a time T.sub.0 required for the secondary ion to travel from the electrode 19 to the drawing electrode 20 is given by EQU T.sub.0 =.sqroot.[m/(2 e n V)] k (5)
Because the distance from the electrode 20 to the secondary electron multiplier 21 is L, a time of flight T.sub.1 required for the secondary ion to reach the secondary electron multiplier 21 from the electrode 20 becomes EQU T.sub.1 =L/u (6)
Eq. (6) can be rewritten, in terms of Eq. (4), as EQU T.sub.1 =L/.sqroot.(2 e n V/m) (7)
A time T required for the secondary ion produced by the sample 18 to arrive at the secondary electron multiplier 21 which is a detector is T.sub.0 +T.sub.1. If, therefore, the time T is measured to determine the voltage V, en/m can be found. In general, where the time at which the ion has been produced is specified, the time-of-flight spectrometry may be more convenient because of its simpler structure.
The SIMS, PSD, and ESD, although they are powerful means for the surface analysis as mentioned above, have individually some problems. Specifically, each of the SIMS and ESD, in which the probe is corpuscular radiation, has the problem that it is basically a destructive analysis means. Furthermore, the SIMS also has the problem that its ion source is unstable and the energy of incidence of the ion beam largely fluctuated. Also, for controlling the shape of a particle beam, such as ions and electrons, it is required to provide a complicated magnetic or electric field type lens and many high-voltage powers of several tens of kilovolts or more. The resultant system is very complicated and high in cost. The PSD, on the other hand, is a nondestructive analysis means and makes the analysis without introducing an effective imaging element in the X-ray region of several hundred angstroms or less. Consequently, the two-dimensional mapping by the microbeam was impossible. Moreover, this spectrometry must use, as a radiation source, a large-scale, expensive SR source, or a mercury-vapor lamp which is low in intensity and emits only the radiation of long wavelengths of several tens of nanometers or more. Thus, the system utilizing such a source had little practical use.